Radar level gauging (RLG) to measure the level of a filling material, such as a liquid or a solid like a granulate is an increasingly important method for level gauging in tanks, containers, etc. Many different types of RLG systems are previously known. An example of such a systems is disclosed in U.S. Pat. No. 7,106,247 by the same applicant.
For many applications, such as for instance in radar level gauging systems for a tanker's load containers, at least one overfill alarm that is functionally independent of the level measuring system is required. Functional independence here means that a fault in one system does not render the other system(s) inoperative. Such independence can be achieved by ensuring that there is no common electrical circuits and cabling, i.e. there must not be any galvanic contact between different measuring systems. However, fixed mechanical constructions for example, which cannot normally go wrong, may be shared. Thus, due to safety restrictions, environmental laws and requirements, etc, there is often a need for several measurements of the surface level of the medium in the container, which measurements are completely separated and independent from each other. The user further may want redundancy in measured values which may require more than one installed measurement system, which may require at least two complete independent level gauges.
Several such radar level gauging system providing two or more functionally independent channels are previously known. For example, U.S. Pat. No. 6,414,625 by the same applicant discloses a device and a method for measuring the level of the surface of a filling material in a container involving more than one radar channel for obtaining a measured value of said level, where the level values measured in said channels are obtained independently of each other. This is accomplished by use of at least two microwave signals distributed in separate channels fed to the same antenna unit, whereby the at least two signals are used to measure the level independently of each other. Said microwave signals are distinguished by means of a detectable characteristic.
A further example of such a system using several channels for feeding one and the same antenna is disclosed in WO 03/025523, also by the same applicant. In this system, the gauging system comprises a feeding mans, which has two or more wave guide members for forwarding the wave guide signals to and from the antenna.
Independent radar level gauges using the same physical antenna but otherwise electrically independent have found use for redundancy at a low cost. The method to connect more than one sensor to one antenna is a very cost effective way to implement e.g. a system with a level sensor and an independent overfill alarm, etc, and has gained wide acceptance among users and authorities. However, previously known systems of this type typically requires complex feedings arrangements, involving complex combiners, couplers and the like, and are therefore relatively complex and costly. Further, these known systems are also impaired by relatively large signal and measurement errors.
For example, the above-discussed system disclosed in WO 03/025523 uses an old turn-style junction as the coupling device to connect the sensors to the antenna, with a coaxial cable to each sensor. The coaxial connectors have due to their design and specifications a rather high VSWR (Voltage Standing Wave Ratio) which introduces an off-set error which is difficult to predict, and in addition the coaxial cables introduce an internal length which will add near-zone errors. Especially for frequencies where standard SMA (SubMiniature version A) connectors cannot be used, typically for frequencies above 18 GHz, the additional cost is also not negligible and the VSWR is bigger.
There is therefore a need for an improved RLG system with two or more independent radar level gauges using the same physical antenna.